Human plasma apolipoprotein A-I has been shown to be polymorphic in plasma and lymph. This polymorphism is due to the presence of proapoA-I and mature ApoA-I as well as amide heterogeneity of mature ApoA-I. Several different structural variants of apoA-I have now been identified. The most important variant is apoA-I-Tangier, which has been isolated to homogeneity. ProapoA-I-Tangier, which is increased in patients with Tangier disease, was shown to have a propeptide sequence which was identical to normal apoA-I. The post-transitional cleavage of proapoA-I-Tangier to mature apoA-I-Tangier was also shown to be normal. The catabolism of apoA-I-Tangier has been previously shown by our laboratory to be markedly increased and the structural change in apoA-I-Tangier which leads to the rapid catabolism is currently being investigated. The complete covalent structure of apoC-II from normal subjects has been determined. The sequence of apoC-II from normal subjects differs from that of apoC-II from some patients with types IV and V hyperlipoproteinemia. The complete solid phase synthesis of apoC-II has also been completed. Synthetic apoC-II has been purified to homogeneity and activates lipoprotein lipase equivalent to native apoC-II. Studies have continued on the isolation and characterization of apoB-100 and apoB-48, the apoB isoproteins secreted by the liver and intestine, respectively. Detailed analysis of the apoB-containing lipoproteins in a patient with apoE deficiency has shown that apoB-100 and apoB-48 are in separate lipoprotein particles. ApoB-48 and apoB-100 can therefore be used as apolipoprotein markers for remnants of intestine and liver lipoproteins, respectively.